III. PHYSICAL AND VEGETATIONAL ASPECTS OF THE SAHARA
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and keep a constant water level for many years. Their supply is provided
either by running water from the surface or from underground reserves
collected in permeable or porous rock formations (sandstone or volcanic
rocks.)
Life in the Saharan oases is based upon irrigation processes (Larnaude,
1937; Capot-Rey, 1953; Planhol and Rognon, 1970). Quite often, the
water at an oasis is collected by various means, but there is always a prevailing utilization process characterizing a whole group of palm groves.
5. Artesian Wells
In the northern part of the Great Oriental Erg, water is supplied by
springs or artesian wells. For example, the artesian system of the Wadi
Rhir is formed by several phreatic underground reservoirs located at different depths; in ancient times, the superior ones are known to have been
used by the natives. In the case of a true artesian water well, the captive
water surges spontaneously out of the ground. Just as a spring can get
dammed up with mud and obstructed, wells will dry out unless they are
cleaned regularly; this is why the famous divers of Ouargla had to dive
into wells as deep as 60 meters or even more in order to disengorge the
springs.
In a semi-artesian well, the ascending movement is not sufficient to bring
up the water to the surface and it has to be drawn out. In most of the
Saharan oases (Souf and Mzab) there are no artesian wells, and these
must be dug to reach the water reservoir. If the water is close enough
to the surface, the well is equipped with a weight lever system (Fig. 12);
if it is deeper a pulley mechanism is then used (Mzab and Fezzan).
Savornin (1927-1928, 1947) was the first to demonstrate the existence of a large fossil water basin, the Albian underground reservoir,
stretching under the entire surface of the Sahara, from the Atlas to the
Hoggar. Today this vast artesian system is exploited by means of mechanical boring devices, capable of drilling to great depths (Cornet, 1964;
Furon, 1964).
6. Irrigation
The most original irrigation method in the Northern Sahara is still the
foggara, typical of the Touat, Gourara, Tidikelt, and Fezzan regions.
In this irrigation process, the water is often collected at great distance
by means of underground, slightly inclined tunnels with several shafts. The
water seeping through the walls is collected and carried by gravity into
the foggara (main tunnel), and from there into the seguia (aerial furrow), opening into the palm grove located downward.
131
and keep a constant water level for many years. Their supply is provided
either by running water from the surface or from underground reserves
collected in permeable or porous rock formations (sandstone or volcanic
rocks.)
Life in the Saharan oases is based upon irrigation processes (Larnaude,
1937; Capot-Rey, 1953; Planhol and Rognon, 1970). Quite often, the
water at an oasis is collected by various means, but there is always a prevailing utilization process characterizing a whole group of palm groves.
5. Artesian Wells
In the northern part of the Great Oriental Erg, water is supplied by
springs or artesian wells. For example, the artesian system of the Wadi
Rhir is formed by several phreatic underground reservoirs located at different depths; in ancient times, the superior ones are known to have been
used by the natives. In the case of a true artesian water well, the captive
water surges spontaneously out of the ground. Just as a spring can get
dammed up with mud and obstructed, wells will dry out unless they are
cleaned regularly; this is why the famous divers of Ouargla had to dive
into wells as deep as 60 meters or even more in order to disengorge the
springs.
In a semi-artesian well, the ascending movement is not sufficient to bring
up the water to the surface and it has to be drawn out. In most of the
Saharan oases (Souf and Mzab) there are no artesian wells, and these
must be dug to reach the water reservoir. If the water is close enough
to the surface, the well is equipped with a weight lever system (Fig. 12);
if it is deeper a pulley mechanism is then used (Mzab and Fezzan).
Savornin (1927-1928, 1947) was the first to demonstrate the existence of a large fossil water basin, the Albian underground reservoir,
stretching under the entire surface of the Sahara, from the Atlas to the
Hoggar. Today this vast artesian system is exploited by means of mechanical boring devices, capable of drilling to great depths (Cornet, 1964;
Furon, 1964).
6. Irrigation
The most original irrigation method in the Northern Sahara is still the
foggara, typical of the Touat, Gourara, Tidikelt, and Fezzan regions.
In this irrigation process, the water is often collected at great distance
by means of underground, slightly inclined tunnels with several shafts. The
water seeping through the walls is collected and carried by gravity into
the foggara (main tunnel), and from there into the seguia (aerial furrow), opening into the palm grove located downward.